Portal Method – Moments and Deflection

The portal method is an analysis method for used for finding approximate internal forces in indeterminate structures subject to a lateral load.

Portal Frame Assumptions:

• Zero moment at the center of each girder. Therefore a hinge is placed here.
• Zero moment at the center of each column, if the column has a fixed base. Therefore a hinge is placed here (or at the bottom of the frame if the frame is hinged at the base).
• The interior columns hinges have twice the shear force as the exterior columns.
  • This is due to the fact that the frame is considered to be a superposition of 2 portals.
  • I also like to envision this due to the fact that the frame acts like a short cantilevered beam where shear is the predominate deformation force. If you recall the shear stress distribution of a rectangular beam, it is greater in the middle of the beam than at the extremes. I admit this may not be techinically correct but it helps me remember the method.

Portal Method Notes:

Portal Method – Approx Deflection Using Energy Methods

I will elaborate on this method in the future. For now take a look at the notes (yes I know the handwriting is awful).

Here is a spreadsheet for single story, pin-base moment frames. Currently it only accounts for bending deflection. Not all that useful but kinda fun none the less.

Portal Method – Lateral Load – Pin Base – Single Story Deflection